Another Project?

[leadlike]

Well, my power transformers arrived-I highly recommend Playthings of the Past-he sent me a photocopy of the datasheet for the one tranny, and heavily labeled the other one to ensure proper installation.

Somehow, I made a mistake, and the one transformer produces a bit more voltage than I really need-350v instead of 240v. So, maybe someone can check my math:

The final B+ needs to be 240v @160ma. My calculations show I need a 687.5 ohm 17.6w resistor to drop those 110v, from a 350v supply voltage.

The B++ line needs to be 315v@85ma. I figured I need a 411 ohm 2.95w resistor to drop that from a 350v supply voltage. Do these figures seem to work?

[Reece]

Re: phono preamp: this must have been one of the sets where G.E. used their new (for the time) variable reluctance pickup rather than a crystal.

[leadlike]

Finally! I think this is the first bit of good news I have had since I started this thing:

Over the past week I mounted my transformers to an upside-down 9x13 cakepan, and began to wire that up, as well as to rewire the tv chassis to accept a large terminal block to wire the power supply into. Well, tonight I finished all of that, and hooked up the filament string only. After a few tests, I was confident enough to place all of the tubes on the chassis, and I was greeted with their heavenly glow:

edit:
sorry no photo, but I am having some net issues.


I would like to continue to power this set up in this manner, a bit at a time, adding on one transformer secondary source after another.

[bandersen]

Cool. Glad to hear that it's starting to come together.

[leadlike]

I got the power resistors today, only there is a single problem: they don't do anything to the voltage. For example:

A few posts up, I figured to drop a 350 volt supply voltage to 240 volts, I would need 687.5 ohms to do the job. Well, putting a 700 ohm resistor on each secondary arm (leaving the center tap to go to ground) gives me about 330 volts (measuring AC from one secondary arm to ground, or the center tap). Removing those resistors, I still have 330 volts. Absolutely no change. Putting on several Kilohms of resistors doesn't change anything, either.

What do I need to do to drop the B+ here? These resistors were the very first thing the transformer secondaries met, before reaching the rectifier circuits or anything else on the tv chassis. I should think these resistors should be able to bake off at least a little voltage, even the electrical engineers at work were on board with my plan. It's all just a bit frustrating.

[bandersen]

Do you have a load attached - how much current ? Your voltage drop will be R x I.

[leadlike]

Oh yeah, the tv was hooked up to it. That should be enough load, I should say.

I found my resistor value by RXI:

for the 687.5, that was found by:

687.5 X .160= 110

the 110 volts is the voltage drop I need (350-240), the .160 is the 160ma that the tv runs under load at the rectifier, and the 687.5 would be the resistance needed. I hope that makes sense.

I even gave this problem to a guy at work, he just started an electrical internship, and is learning the basics of DC. I simply gave him the current and the voltage needed to be dropped, and he got the same answer. He was happy to be working with a "real" problem, rather than one on paper.

[bandersen]

Sorry, I didn't remember your earlier post where you layed out your plan.

Sure, it makes sense. The math is sound so something's screwy here. Did you measure any voltage drop across the resistors? Did they get warm?

You could try adding a single dropping resistor after the rectifier instead.

[7"estatdef]

I had a passing thought that my old '48 Admiral consolette uses a transformer similar to that. I don't remember the specs on it but it does have the dual hv windings. Seems like it has about 270v and 155v output at the currents you are looking for.
Terry

[leadlike]

I found the problem-the choke after the rectifier is open. AAAAHHHH!!! Anyway, I have a few that will meet the current rating, but none that have as much resistance or inductance as the original (500 ohm, 7H). I think I'll have to make a run to storage and pull my T-120 chassis and see if that choke will be a good match.

I found that when I disconnected the bad choke, some voltages went down, while others went up. It must have been loading things down. I'll have a better idea once I get a new choke in.

[leadlike]

A little light at the end of a very dark tunnel....B+ is a few volts low, B++ is dead on. The flyback (although this is an iron core transformer, so is this still a flyback?) is putting out just under 10kv, the schematic calls for 8.3kv. Vertical is shrunken, but the controls are very dirty...perhaps a deoxit will clear all of that up.


GEDC0139.jpg

[bandersen]

Awesome It's great to see that CRT glowing!

[leadlike]

I just can't seem to get anywhere with this set. The Phono section seems to be working (though I would need the proper phono cartridge to be sure). The AM radio performs superbly, but I cannot get anything on FM, or audio or video on any of the television settings.

All of the voltages are in line as they should be, but I just don't own any television alignment equipment except for a color bar generator. I can inject audio and video through all of the stages, but when I try to inject any rf into any stage, I barely get anything-probably just swamping the receiver, as I get this same weak signal on all channels.

I have added all new tubes into the rf/if sections, cleaned the tube sockets, cleaned all of the controls, and measured continuity across all coils. I just have no clue where to go from here-save for adding a/v jacks and bypassing the tuner entirely.

[Jeffhs]

I'm curious. If the present owner of this TV already has a modern flat-panel set he wants to put on top of the cabinet, what would he need the original TV for? The only thing I can come up with is that he may want to feed the audio from his home theater setup through the original TV's sound channels, but that doesn't make much if any sense since all home theater receivers have outputs for multiple speakers (some as many as seven, counting subwoofers).

Also, there is the problem of power line isolation. I personally wouldn't try to use a 70-year-old TV with any audio system having solid-state components, unless I were sure beyond the shadow of even the most unreasonable doubt that the isolation capacitors were good--most such caps in unrestored sets of this age all but certainly failed years or decades ago, and cannot be trusted. Even if the original isolation networks were replaced with new components, I would still be leery of such things as ground loops or voltage on the chassis of the modern device.

[leadlike]

Jeff,

The owner's ideal plan would be to use the set to watch B&W films on a few times a month. The flat panel would be the "daily driver" for all of his other programming.

They would be hooked up separately, sharing a cable box (a/v outputs going to the flat panel, coax out going to an rf modulator for the GE). If I did have to add a/v inputs, you can be sure that even though the coupling caps are all replaced, I would still add a cap to the inputs, to further isolate any solid state gear that could be feeding it.

A few neat items on circuit design in this set:

I found the plate voltages to be 50v too high on all of the video IFs. It turns out that the contrast control is actually a gain control. As you adjust the contrast, you can alter the plate voltage of all IF sections by about 60v or so.

The vertical and horizontal hold controls are in the rear of the set. The literature I have been reading on this set describes these controls as something that only need be set up once, and then left alone. We'll see how much that is true once the set it running. I will say that the vertical height, hold, and linearity interact with each other a good deal. If that vertical hold control was on the front of the set, in the hands of the average Joe, you could probably really mess up the picture.